Using Fermi Large Area Telescope Observations to Constrain the Emission and Field Geometries of Young Gamma-ray Pulsars and to Guide Millisecond Pulsar Searches

Using Fermi Large Area Telescope Observations to Constrain the Emission and Field Geometries of Young Gamma-ray Pulsars and to Guide Millisecond Pulsar Searches

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2013

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DeCesar, Megan

##### Advisor

Miller, Michael C.

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##### Abstract

This thesis has two parts, the first focusing on analysis and modeling of high-energy pulsar emission and the second on pulsar observations. In part 1, I constrain the magnetospheric emission geometry (magnetic inclination α, emission width <italic>w</italic>, maximum emission radius <italic>r</italic>, and observer colatitude ζ) by modeling >100 MeV light curves of four bright γ-ray pulsars with geometrical representations of the slot gap and outer gap emission models. I also model the >100 MeV phase resolved spectra, measuring the power law cutoff energy <italic>E</italic><sub>c</sub> with phase. Assuming curvature radiation reaction (CRR) is the dominant emission process, I use <italic>E</italic><sub>c</sub> to compute the accelerating electric field strength, <italic>E</italic><sub>||</sub>.
The original contributions of this thesis to astrophysical research are the use of the force-free magnetic field solution in light curve modeling, the inclusion of an offset polar cap in the slot gap geometry, and the calculation of <italic>E</italic><sub>||</sub> from observationally determined quantities (i.e., <italic>E</italic><sub>c</sub>).
The simulations reproduce observed light curve features and accurately match multi-wavelength ζ measurements, but the specific combination of best-fit emission and field geometry varies between pulsars. Perhaps pulsar magnetospheres contain some combination of slot gap and outer gap geometries, whose contributions to the light curve depend on viewing angle. The requirement that, locally, <italic>E</italic><sub>||</sub>/<italic>B</italic> < 1 rules out the vacuum field as a valid approximation to the true pulsar field under the CRR assumption. The <italic>E</italic><sub>||</sub> values imply that the youngest, most energetic pulsar has a near-force-free field, and that CRR and/or narrow acceleration gaps may not be applicable to older pulsars.
In part 2, I present discoveries of two radio millisecond pulsars (MSPs) from LAT-guided pulsar searches. I timed the first MSP, resulting in the detection of γ-ray pulsations. The second MSP is in a globular cluster. My initial timing efforts show that it is in a highly eccentric (<italic>e</italic> ~ 0.95) binary orbit with a massive (>0.7M<sub>sun</sub>) companion, suggestive of past companion exchanges and an exotic nature of the current companion. Further timing will yield a measurement of the orbital precession rate and the system mass, yielding neutron star mass constraints.